1. Field of the Invention
The present invention relates to a holding sealing material, a method for manufacturing a holding sealing material, and an exhaust gas purifying apparatus.
2. Discussion of the Background
Particulate matters (hereinafter, also referred to as PMs) are contained in exhaust gases discharged from internal combustion engines such as diesel engines, and in recent years, there has arisen a serious problem that these PMs are harmful to the environment and the human bodies. Moreover, since exhaust gases also contain toxic gas components such as CO, HC, and NOx, there have been growing concerns about influences of these toxic gas components on the environment and the human bodies.
In view of these, as an exhaust gas purifying apparatus for collecting PMs in exhaust gases and for purifying the toxic gas components, various exhaust gas purifying apparatuses have been proposed. Each of the exhaust gas purifying apparatuses is configured by: an exhaust gas treating body made of porous ceramics, such as silicon carbide and cordierite; a casing configured to accommodate the exhaust gas treating body; and a holding sealing material made of an inorganic fiber aggregated body that is disposed between the exhaust gas treating body and the casing. This holding sealing material is installed mainly for purposes of preventing the exhaust gas treating body from being damaged upon contact with the casing that covers its periphery because of vibrations and impacts caused by traveling or the like of an automobile and for preventing exhaust gases from leaking between the exhaust gas treating body and the casing.
Here, since the internal combustion engines are operated at an air fuel ratio close to the theoretical air fuel ratio for the purpose of improvement in fuel consumption, exhaust gases tend to increase their temperature and pressure. When exhaust gases having a high temperature and a high pressure reach the exhaust gas purifying apparatus, the difference of coefficient of thermal expansion between the exhaust gas treating body and the casing leads to variations of the interval therebetween. Therefore, the holding force of the exhaust gas treating body that does not change with some variations of the intervals is required of the holding sealing material. In order to allow the exhaust gas treating body to effectively exert exhaust gas treating properties, there has been a growing demand for a holding sealing material having heat insulating property to keep the exhaust gas treating body warm.
For the purpose of fulfilling these demands, there has been employed, in recent years, a designing method to increase the thickness of the holding sealing material and thereby improve heat insulating property. In such a holding sealing material, it is necessary to increase the unit weight of the holding sealing material for securing the repulsive force of inorganic fibers which is a factor of the holding force.
However, it becomes difficult to obtain sufficient peel strength in the needling treatment to be performed in the manufacturing process in order to raise the peel strength in a thickness direction, as the thickness of an inorganic fiber aggregate is increased. Accordingly, when an exhaust gas treating body around which the holding sealing material has been wound is press-fitted into a casing, marked shear deformation of the holding sealing material and the like may be caused.
On the other hand, a holding sealing material has been proposed which exhibits a higher weight by combining a plurality of mats having the same weight as the weight of conventional mats instead of changing the thickness of each of the holding sealing materials. There is disclosed a holding sealing material, which is configured by laminating a plurality of heat-resistant mats, which makes it possible to wind each of the mats without any looseness when each of the mats is wound around an exhaust gas treating body while laminated, and which has a length set so as to allow the engaging portions formed on both end portions to engage each other (JP-A 2007-218221).
Here, in the holding sealing material of JP-A 2007-218221, when a plurality of mats are laminated, a fixed portion is provided by machine-sewing in order to prevent the problematic displacement in a width direction of each of the mats.
The contents of JP-A 2007-218221 are incorporated herein by reference in their entirety.